This invention relates to semiconductive devices. It is of primary application to the fabrication of such devices which involve a surface barrier (Schottky) connection.
Modern semiconductor technology has been making increasing use of Schottky connections both in discrete devices such as Schottky diodes and in integrated circuit forms such as Schottky TTL logic.
One of the attractions of Schottky connections is that it permits a rectifying barrier which has a lower threshold voltage for significant conduction in the forward direction as compared to p-n rectifying junctions. Such low threshold devices are becoming of increasing interest, particularly in microelectronics where operation is advantageously at low voltage signal levels. The threshold voltage associated with a surface barrier connection is dependent both on the metal used in making the connection and the nature of the underlying semiconductor where the connection is made.
While one expedient for getting a low barrier connection is choice of an appropriate contact metal, this is not always an adequate solution, particularly when the choice of contact metal is dictated by other considerations, such as the case when the same metal is to be used to make a number of separate connections of different properties.
It has been suggested hitherto that the effective height of a surface barrier connection for a given metal and correspondingly the threshold voltage can be reduced by a shallow implant of appropriate dopant ions in the region of the semiconductor underlying where the surface barrier connection is to be made. In particular, the implantation is used to create there a localized layer of higher doping than the substrate to lower the effective barrier height. Moreover, essentially all of the implanted ions are sought to be localized in a shallow surface layer to be encompassed within the depletion layer associated with the contact metal at zero applied bias. It is important to maintain the bulk of the substrate at lower doping in order to keep the reverse leakage low. For a discussion of this technique, see U.S. Pat. No. 3,943,552 which issued to J. M. Shannon et al on Mar. 9, 1976. This technique is also discussed in a paper by J. M. Shannon entitled "Reducing the Effective Height of a Schottky Barrier Using Low-Energy Ion Implantation" in Applied Physics Letters, Vol. 24, No. 8, Apr. 15, 1974. In practice, we have found it extremely difficult by prior art techniques, to achieve the desired reduction in threshold voltage in metal-silicide devices without also increasing undesirably the high reverse leakage associated with such a connection. One problem appears to be difficulty in actually limiting most of the implanted ions to the desired shallow layer. In particular, it is relatively difficult to get an ion beam to produce the required narrow impurity distribution immediately below the platinum silicide-silicon interface. In addition, very shallow implants are particularly sensitive to the nature of the surface.